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Immature erythroblasts undergo extensive self-renewal ex vivo when derived from the early embryo. A day 9.5 mouse embryo with yolk sac attached and Wright-Giemsa–stained extensively self-renewing erythroblasts are pictured. Definitive erythroid precursors derived from the yolk sac and early fetal liver of the mouse embryo are capable of more than 10³⁰-fold self-renewal divisions in ex vivo culture, while maintaining the ability to mature into enucleated erythrocytes. Similar erythroid cells derived from adult erythroid progenitors are capable of much more limited self-renewal. See the article by England et al on page 2708.